The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string, having a drill pipe and a drill bit at its lower end, against the earth. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid composition down through the drill string and up to the surface between the drill string and the earthen wall of the borehole for the first few hundred feet. Native muds made of, for example, seawater and sediment are usually sufficient for cooling the drill bit and removing the cuttings while drilling a shallow surface hole.
Once the borehole has been drilled to the first few hundred feet, casings are usually inserted into the borehole extending from the surface downward in order to isolate the separate areas, zones or formations transversed by the borehole, and for protection against borehole collapse. The drill string is removed from the borehole, the casing is lowered into the borehole with each 30 ft segment typically being screwed onto the 30 ft segment below it. Once the desired length of casing has been inserted into the borehole, the drill string, made of a hollow pipe onto which is attached the drill bit, is lowered down the casing to continue drilling operations. Thus, a typical configuration would comprise, from outward in, the earthen borehole wall, the casing supporting the borehole wall, and the hollow drill string within the casing. Since the casing has a smaller diameter than the borehole wall, and the hollow drill string has a diameter smaller than the casing, there exists an annulus between the borehole wall and the casing, and between the casing and the drill string.
As the drill string within the casing rotates to bore the earth, a drill mud is pumped down the hollow drill string pipe, out the drill bit, and up through the annulus between the drill string and the casing to the surface, where it is treated and recirculated back down the drill string. The drilling mud has many functions. The drilling mud functions to carry chips and cuttings produced by drilling to the surface; to lubricate and cool the drill bit and drill string by carrying away heat; to form a filter cake which obstructs filtrate invasion in the formation; to maintain the walls of the borehole; to control formation pressures and prevent lost returns; to suspend cuttings during rig shutdowns; and to protect the formation for later successful completion and production. Further, since downhole temperatures can reach 200.degree. C., the mud should retain its non-Newtonian characteristics within a wide range of downhole temperatures to successfully suspend the cuttings during temporary shutdowns, such as when more casing is lowered.
As the drilling depth increases, it becomes necessary to change the types of muds being pumped down the drill string. For example, for shallow drilling, water based muds usually possess sufficient properties to perform the necessary functions demanded of a mud. However, as the drill depth increases and one proceeds from one formation to another, so does the formation pressure and the heat generated by the drill bit. Since formation pressures increase at greater depths, it becomes necessary to use a higher weighted mud. Eventually, the mud has to be changed to a weighted oil based mud, also known as water in oil emulsions or invert drilling muds, to properly lubricate the drill bit, control formation pressure, and carry the cutting away from the drill bit and out the casing without breaking down. To prevent the new type of mud from contacting and becoming contaminated with the mud in use, a displacement fluid composition (a spacer fluid composition) is typically injected down the drill string behind the mud in use and in front of the new mud. The spacer fluid composition provides a physical barrier between different types of muds to avoid contact, and also serves to clean the hardware of the old mud.
Once the total drill depth is achieved, it becomes necessary to fill the annulus between the casing and the borehole wall with a cementitious material which will seal the annulus to inhibit communication between various formations penetrated by the wellbore and which will provide structural support for the casing or liner. This is commonly referred to as primary cementing. This is accomplished by removing the drill string, and pumping cement down the casing to the annulus opening between the casing and the earthen borehole, and up this annulus to the surface to cement the casing to earthen borehole. In order to perform the cement job, it is first necessary to displace the mud in the drill string and in the casing because conventional Portland cement and conventional drilling muds are incompatible. Also, contamination of the drill string and casing by the drilling fluid composition residue, especially oil based drilling fluid compositions, reduces the adhesion of the cement to the casing, leading to fluid and gas leaks. Further, as the cement is forced down the casing and up into the annulus, it is commingled with the drilling mud at any interface between the mud and the cement. The resulting mixture generally becomes a gel and does not set up into a strong cement. In addition, the gel strength and viscosity become uncontrollable and the mixture may either become too viscous to pump or may get thinner. In either event, the situation is unsatisfactory.
Therefore, a displacement fluid composition is used to displace the mud in the drill string and the casing prior to pumping cement down the casing. The displacement fluid composition acts as a physical barrier between the mud and the cement and cleans the hardware to avoid contaminating the cement with the mud. A displacement fluid composition is pumped down the casing through a hollow pipe inserted into the casing to remove and clean the mud from the inner lining of the casing and any hardware which may have contacted the mud. Then the cement is pumped down the casing and up the annulus between the casing and the borehole. The wellbore is then ready for completion operations, which usually consists of circulating other cleaning fluids through the casing, such as a gel spacer, a brine, and a solvent wash. Each of these fluids also operate to displace the fluid ahead of it already downhole. At this point, the casing is ready for a final completion operation, which normally consists of injecting and circulating a dense completion fluid composition down the casing to control formation pressures, optionally in a packer at the productive formation zone, and detonating an explosive device with, for example, steel bearings to perforated the casing and the wall of the borehole, thereby allowing the gas or crude hydrocarbons to penetrate through and up the casing.
The displacement fluid composition should exhibit several properties. The displacement fluid composition should be a good cleansing agent to remove oily layers and films from the hardware, thereby preventing contamination between one mud and another mud, and more importantly, prevent contamination of the cement with the oil based drilling mud. In particular, it is desirable that the rate of emulsification of the oil based mud be as fast as possible to reduce the amount of displacement fluid composition needed and/or reduce cycle time. It is also desirable that the displacement fluid composition have a high emulsification capacity so as to remove and stably suspend large quantities of oily cuttings, particulates. The displacement fluid composition should also be biodegradable to protect the environment surrounding the drilling operation, whether land or offshore based. Other desirable properties of displacement fluid composition are that they exhibit fluid composition-loss control, favorable rheology, and favorable density to control formation pressures.
Particularly unfavorable hydrocarbons used in drilling muds at one time were the diesel based fluids. These fluids were only poorly biodegradible. Other types of fluids used in drilling muds ranged from ester based fluids and polyalphaolefins, each of which were unsatisfactory either with respect to biodegradibility or in performance.
It had been a common practice to use an ethoxylated nonylphenol surfactant in spacer fluid compositions and even in drilling muds. For example, U.S. Pat. No. 4,717,488 describes a spacer fluid composition comprising a heteropolysaccaride S-130 (i.e., Biozan TM manufactured by Merck & Co., Inc.), an organophilic clay, a hydrocarbon solvent, a surfactant such as an ethoxylated nonylphenol surfactant or an ethoxylated linear alcohol, and an optional lower alkanol. U.S. Pat. No. 5,113,943 describes a spacer composition having a sulfonated styrene maleic anhydride copolymer, water, weighting agents, and an ethoxylated nonylphenol surfactant having a mole ratio of ethylene oxide to nonylphenol in the range of 1.5 to 15. U.S. Pat. No. 4,588,032 also describes spacer fluid composition compositions containing a mixture of nonylphenol ethoxylates having 1-6 moles of ethylene oxide and 7-14 moles of ethylene oxide, optionally in combination with sulfonated (anionic) linear alcohol ethoxylated with 2-20 moles of ethylene oxide, and further optionally in combination with a 3-8 carbon alcohol ethoxylated with 2-4 moles of ethylene oxide.
Ethoxylated nonylphenols are generally poorly biodegradable, and therefore their use in drilling fluid compositions, muds, or spacer fluid compositions is problematic to the environment, especially under the ocean floor where leakage is difficult to contain or clean up. Therefore, it would be desirable to formulate displacement fluid composition which do not rely upon the use of ethoxyalted nonylphenols as the surfactant. It would also be desirable to use a surfactant base which is non-ionic to avoid foaming tendencies commonly associated with the anionic sulfonate surfactants.
Nonionic aliphatic alcohols oxyalklyated with ethylene oxide have been described as one of the useful surfactants in spacer or drilling fluid compositions, such as the above mention U.S. Pat. No. 4,588,032 patent, as well as in U.S. Pat. No. 5,330,662. While such surfactants are usually readily biodegradable, their emulsification performance is less than desirable. In particular, not only should the surfactant be readily biodegradable, but its rate of emulsion and the emulsion capacity should be as high as possible, at least higher than that of a nonionic aliphatic alcohol ethoxylate or an ethoxylate of nonylphenol.